Project Summary The mammalian host requires CD4+ T cells to survive infections with many distinct intracellular pathogens. Loss of CD4+ T cells leads to susceptibility to infections caused by Chlamydia trachomatis, Mycobacterium tuberculosis and others. Antigen presenting cells (APCs) are at the interface of invading pathogens and adaptive immunity, sensing the infected environment and signaling directly to CD4+ T cells. The expression of Major Histocompatibility Complex II (MHCII) on APCs is critical for CD4+ T cells to detect their cognate antigens and activate an appropriate host response. Many pathogens manipulate the expression of MHCII to evade protective immunity. While complete loss of MHCII prevents CD4+ T cell function, how modulation of MHCII levels during infection impacts the effectiveness of pathogen-specific CD4+ T cell responses remains unknown. We hypothesize that MHCII expression levels are an important factor in the activation and effector function of antigen-specific CD4+ T cells. By understanding how MHCII is regulated at a global scale we will be positioned to develop interventions that modulate MHCII expression in order to overcome pathogen virulence strategies aimed at blunting protective host responses. We performed a genome-wide CRISPR-Cas9 screen to identify regulators of MHCII surface expression in activated APCs. We found over 200 genes that significantly alter surface MHCII expression with no described function in MHCII regulation. Here, using CRISPR-Cas9 mediated gain-of-function and loss-of-function approaches we will systemically identify priority candidates that enhance or inhibit MHCII surface expression and begin to dissect their mechanisms of MHCII control. We will next determine how modulation of MHCII expression directly alters the pathogen-specific CD4+ T cell response during genital infections with Chlamydia muridarum. We are investigating Chlamydia because it is an important pathogen clinically as the number one bacterial sexually transmitted infection in the United States. Chlamydia causes severe pathologies that lead to infertility, ectopic pregnancy and pelvic inflammatory disease. We will track the Chlamydia-specific CD4+ T cell response using immunological tools that do not exist for most pathogens. In combination with our CRISPR approaches we are uniquely positioned to directly examine how MHCII expression impacts the Chlamydia-specific CD4+ T cell response ex vivo and in vivo. We will determine how enhanced or reduced MHCII expression on APCs alters a range of CD4+ T cell phenotypes including activation, proliferation, effector function, protection and the development of genital tract pathologies. Our focused studies on the role of MHCII expression during C. trachomatis infection will broadly identify regulators of MHCII that can then be explored in a range of disease states where CD4+ T cell activation is required and MHCII is manipulated including other intracellular pathogens or cancers.